The invention relates to an adaptive equalizer which performs an equalization of a digital input signal transmitted by a channel. Such an equalizer comprises a combination of at least a summer and a digital linear filter B controlled by a clock of period T. The filter B comprises delay cells and calculation cells which apply weighting factors.
The above-described equalizer may be used for correcting distortions produced by a channel while the signal is being transmitted. This may relate to an echo canceller. The transmission may be realized by air or by cable.
The invention may also relate to television signals, or coded or uncoded digital signals.
The possibility of multiple paths when signals are transmitted in the microwave frequency range leads to an echo signal generation. Between an echo and an original signal, short or long delays may occur (which may reach 10 microseconds) with short and/or long echoes. Similar situations occur for digital transmissions, for example, with microwave links.
For performing an equalization of the signals in such situations, an echo canceller is known which is described in the Patent Abstracts of Japan, Vol. 5, No. 23, 12 Feb. 1981, relating to Japanese Application No. 55-149524. That discloses an equalization system which improves operating stability and equalization performance by combining a first filter and a second filter to produce a filter which may be switched between a direct type of operation and a recursive type of operation with the aid of a switch. The second filter is associated with a delay element so that the second filter corrects echoes that have long delays. Due to the switching operation, there is a sudden transition between the two operating modes of the filter.
A same filter having the same coefficients cannot maintain identical operation if it consecutively operates in the direct mode and in the recursive mode. In effect, for a given filter length, a recursive linear filter performs a better filtering than a direct linear filter. But a recursive filter may become unstable under specific circumstances, which is not the case with a direct linear filter. Conversely, a recursive filter intrinsically needs fewer coefficients, thus, fewer multipliers, and also fewer delay cells to obtain a similar filtering performance. Such filters are, thus, much more compact and less costly.
According to the above-mentioned Japanese Patent Abstract, there is a sudden performance degradation when a switching operating is performed in which the switch changes the filter to a direct mode of operation. If, for example, transmitted television pictures are processed in this manner, a mediocre picture quality will result. Thus, a better mastering of the operation of such an echo canceller is to be preferred. In effect, in the varied conditions in which these filters are intended to be used, the filtering quality is to remain excellent. This is especially the case for the equalization of television signals transmitted by a microwave channel. Any unstable situation must then be avoided even temporarily.